High‐Titanium Zircon Rims in Rhyolite – A Reflection of Adiabatic Ascent?

ABSTRACT Autocrystic zircon formed during magma crystallisation provides crucial information for understanding the long‐term evolution of Earth's crust. Due to the factors that control zircon solubility in silicate melt (e.g., temperature and melt composition), zircon crystallisation may occur during magma ascent, emplacement, and eruption. Zircon sourced from volcanic rocks can have rims with elevated concentrations of titanium relative to zircon cores; this has been linked to temperature changes associated with recharge events in magma chambers. An alternative cause of temperature shifts in subvolcanic systems is adiabatic ascent without magma mixing or recharge. Here, we simulate adiabatic ascent of rhyolitic magmas to model the growth and dissolution of zircon during ascent from the mid‐crust to sub‐volcanic depths – decompression melting and system expansion drive cooling prior to fluid saturation. Sub‐volcanic boiling induces magma crystallisation. Zircon growth is predicted throughout ascent for both A‐ and S‐type magmas. Our results suggest that high‐titanium rims of autocrystic zircon can form through the protracted evolution of a single magmatic pulse without the need for magmatic recharge.